The typical "next big thing" is sought from those who have run fast on low mileage, the theory being that as they run more their natural speed will get a better engine and they will progress. But that doesn't always happen - sometimes they can't handle the mileage and sometimes the mileage just doesn't make that much of a difference.
Another possibility is any kids that have already run high mileage but not done much structured training yet - kind of like the Kenyans, but on a lower scale, I guess. I met a guy on a training run the other day whose son ran with his wife and him from a very young age and ran a 1:20 half at 13 having done very little speedwork or structured training, just enjoyed getting out on the trails with his parents and siblings. He doesn't like running fast so much yet (hurts more) but is starting to get more competitive and wanting to train harder more and more as time goes by. Sub 17 5K I think (14 now?).
This is more unusual for sure, but where it exists could bring a few surprises out of "nowhere" (where "nowhere" is a decade of unstructured aerobic base)